|
In cryptanalysis, a crib is a sample of known plaintext, or suspected plaintext; the term originated at Bletchley Park, the British codebreaking operation during World War II (WWII). Their usage was adapted from the term meaning a bit of a cheat, for example, "I cribbed my answer from your test paper". The original sense of a "crib" was a literal or interlinear translation of a foreign language text — usually a Latin or Greek text — that students were likely to be assigned in the original language. Examples include stereotyped salutations, endings, titles, routing codes, etc. In the case of WWII German traffic, one site regularly reported to headquarters each morning using precisely the same phrase, albeit enciphered using the current Enigma key. Bletchley Park sometimes arranged operations to provoke messages from the Germans to which they knew the plaintext; this was termed gardening. The British were greatly helped in finding cribs by a German Commander in the Afrika Korps who tended to send a "nothing to report" message to his headquarters most days. Because the British learned of this, they were able to watch for this (frequent) signal and use it as a helpful crib for determining the day's encryption key. Another aid to finding useable cribs was the fact that although German Enigma operators were under instructions to use random letter sequences as encryption keys when preparing to encrypt a message, a few of them in fact chose obvious patterns on their Enigma keyboards, such as QWE or MKO. Knowing this sometimes allowed the British to determine part of the key more quickly. These "lazy" setup keys were termed Cillies (a misspelling of "silly"). Yet another crib-finding technique was known as the Herivel Tip after the person who suggested it (named John Herivel). It had to do with the fact that after setting up the letter-pointer ring on each Enigma codewheel, the operator often didn't bother to rotate them before placing them into the machine, but simply slid all of them down into their slots at or close to the positions they had just had in his hands.
| ||||||||
|
--> | |||||||||
 |
|
| |